The rainy season brings us things we don't see much of throughout the year -- clear blue skies, emerald green hills, silver snow-capped mountains. It also resurrects a relic of Southern California's rural past: the vernal pool.

North County and Southwest Riverside County are home to some of the best remaining examples of the seasonal pools that emerge in winter, then gradually fade into rings of brilliant wildflowers in the spring sun.

And now is as good a time as any to view the pools, which are becoming fewer and fewer.

"I think they're amazing," said Mike Land, a medical customer services representative who lives in San Marcos and hikes local trails. "I don't think there is anything else in the world like the vernal pools."

Much more than big mud puddles, vernal pools are seasonal bodies of water that spring up after a series of intense, wet storms and hang on for weeks or months.

The pools are home to a rich mix of plants and animals.

"There are so many unique species that are found nowhere else, both plants and animals, around vernal pool habitats," said Kay Madore, a volunteer docent who monitors pools on the Santa Rosa Plateau.

"Think of them as little islands," said Fred Roberts of Oceanside, a rare-plant analyst for the San Diego chapter of the California Native Plant Society. "The islands off our coast have unique species on them. It's the same thing here."

Roberts, who worked for the U.S. Fish and Wildlife Service from 1991 to 1999, said the pools, which can be as small as a couch or as big as a pond, are "almost like a natural bathtub."

They form in shallow natural depressions underlined with clay or rock that blocks water from seeping through, Roberts said.

Because they are temporary, fish cannot live in them. That enables tiny freshwater shrimp -- such as the San Diego, Riverside and vernal pool fairy shrimp -- to thrive, though some are eaten by birds and beetles.

Only a few left

As pools recede and the air warms, the shrimp lay eggs and die. Then rare plants that tend to grow only around vernal pools take over.

Examples include several types of Brodiaea, with their blue and lavender flowers, the gold-and-lavender Downingia cuspidata (bellflower) and the white star-shaped spreading Navarretia (pincushion plant), according to a report by Wayne Armstrong, a Palomar College professor of biology and botany.

Shrimp eggs lie in the soil unharmed throughout the long, hot summer, waiting for an infusion of rain to trigger their metamorphosis into fairy shrimp. Similarly, plant seeds lie dormant in the soil waiting for the next cycle of nourishing moisture and sun to convert them into colorful wildflowers.

"It's a unique little habitat," Roberts said. "And it's found only in Mediterranean (climate) regions."

But it is a habitat that can be found now only in few places around Southern California, as 95 percent of the region's pools have been destroyed by the blade of the bulldozer, scientists say.

For that reason, some local biologists oppose San Diego County's plans to build a 20,000-square-foot branch library at 13th and Main streets in Ramona, on 7.2 acres that are home to a half-dozen tiny pools. Because the pools would disappear, the U.S. Army Corps of Engineers is seeking public comment through March 24 before issuing a required permit for the project.

Comments are being accepted by mail at Los Angeles District, Corps of Engineers, San Diego Regulatory Field Office, Attn: Terry Dean, 6010 Hidden Valley Road, Suite 105, Carlsbad, CA 92011. Comments are being taken by e-mail at Terrence.Dean@usace.army.mil.

County officials plan to make up for the loss by expanding a vernal pool complex at Ramona Airport.

Roberts questioned whether the strategy will work, saying it is difficult to duplicate the soil structure of vernal pools.

"In many ways, it is far easier to leave the existing one in place and put a fence around it," Roberts said.

But in this case, said Dahvia Lynch, environmental project manager for the county Department of General Services, the site is in an urbanized area -- downtown Ramona -- and the pools are vulnerable to fuel spills and trampling by people.

"It's one of those cases where it just doesn't make sense to preserve," Lynch said.

Still, Roberts contends such areas should be treated differently than other habitats.

"There are so few vernal pools left," he said. "That's the one habitat that we should have virtually no tolerance for development on."

Up close and personal

In many cases, putting a fence around pools and leaving them alone is exactly what local officials and landowners have chosen to do.

That was done with a pool behind the Fry's Electronics store in San Marcos. And it was done with the Skunk Hollow vernal pool in French Valley, one of the biggest at 33 acres.

Despite those barriers, the San Marcos and French Valley pools are easy to see.

Many other pools, such as the 2,400 or so on Camp Pendleton Marine Corps Base and the 3,000 on Miramar Marine Corps Air Station, are hidden from public view.

From time to time, public areas close, too.

Jason Price, manager of the 80-acre Del Mar Mesa Ecological Reserve off Highway 56 at Rancho Penasquitos, said the pools there were closed to foot traffic last week because of damage from illegal off-roading activity. But Price said the area will reopen once the area has had time to heal.

Rare is the place where one can view a pool up close. But there are still a few.

One is the Poinsettia Lane Coaster train station in Carlsbad.

Another is the 8,300-acre Santa Rosa Plateau Ecological Reserve near Murrieta. There, a pond 16 inches deep and spanning 22 acres sits atop a slab of volcanic rock. It has had water since December and is brimming after the recent rain.

A fairy tale

"Because the pool formed early, the vernal pool fairy shrimp has already lived its life cycle and is no longer in the pool," Madore said, adding that biologists are watching closely to see if the pond will breed a rare second crop of shrimp in the same season.

As their name would suggest, fairy shrimp are among Southern California's more unusual animals.

They have elongated bodies a half-inch to an inch long, big, out-of-proportion eyes and 11 pairs of legs. To swim, they flip upside down and beat their legs in a wavelike motion.

They live 40 to 50 days. They eat, mate and die, leaving behind new eggs, or cysts.

Unlike true eggs, the cysts have legs, eyes and tails, and when they hatch, they produce fully formed shrimp.

Cysts lie in the soil year after year, surviving summer heat and winter wetness before hatching. They have been known to lie dormant as long as a century and still generate living fairy shrimp.

Of course, the fairy tale ends every year when the pools dry up. That can come as early as the season's last storm or as late as July.

This year, Madore said, the plateau pool should last until mid-June.

Contact staff writer Dave Downey at (760) 745-6611, Ext. 2623, or ddowney@nctimes.com.